Constant velocity or CV joints, as used on automotive drive axles, such as on front wheel drive cars, are commonly enclosed within a convoluted elastomer boot. Such convoluted boots, that is, boots having a plurality of axially spaced annular convolutions, are commonly formed with a small end which is retained by a clamp on the drive shaft leading out of the joint, and a large end clamped to an annular surface of the body of the CV joint. Such boots provide a seal for the joint over the operating range of the constant velocity joint, which may include drive offset angles from between 0.degree. up to 40.degree. or more between the shaft and the joint body. Such boots are commonly molded of elastomeric material, such as rubber, which may have a hardness in the range of Shore 65A, for example, which permits the boot to flex with the flexing of the joint. The softness of the rubber does not result in the placement of undue stresses on the retaining clamps. Typically, the encircling clamps are of a low profile design, so as to have relatively low weight, and provide the compressive force sufficient to seal the ends of the boot to the shaft and CV body and to resist sliding or movement of the boot under the clamp when the CV joint is operated at high angles.
More recently, blow molded boots formed of more rigid thermoplastic polyester elastomers have come into use as a substitute for the rubber boot, by reason of the greater mechanical and physical strength of such elastomers over a wide variety of operating conditions, and generally by reason of the greater toughness of such elastomers and their ability to resist puncture or tearing. However, such blow molded polymers are considerably harder and stiffer than their rubber counterparts, and may, for example, have a hardness in the range of Shore 50D and a flexural modulus of 18,009 psi or more (at 22.degree. C.). Such relatively stiffer boots impose substantially greater demands on the retaining clamps, requiring substantially greater compressive clamping forces to hold the large end of the boot in place on the body of the CV joint, without movement under the clamp, while still providing an effective seal between the boot and the body.
Typical materials from which blow-molded convoluted boots have been made are commonly referred to as thermoplastic elastomers (TPE) and include polybutylterephalate compounds (PEEBT). Typical thermoplastic elastomer materials which are used for blow-molding convoluted boots, as defined above, include du Pont "Hytrel", HTG-5612 and Monsanto's "Santoprene" thermoplastic rubber, typically grade 103-40. Such elastomers, as compared to typical rubbers, have a substantially greater tendency to deform at relatively low elongation forces, and typically have a maximum or 100% modulus which is less than half of the ultimate tensile stress.
The relatively high stiffness of such blow-molded polymers normally suggests the employment of a correspondingly heavier clamp with high clamping forces in order to hold the boot in place on the body, particularly when the boot is operated under angular offset conditions, such as on a constant velocity joint. This is especially the case at the large end of the boot, where the large inside diameter is clamped to the joint housing. In this instance, a combination of dissimilarities combine to make more difficult the clamping of such a convoluted boot, particularly at its larger end. These include (a) a greater force required to compress TPE material over its rubber counterpart, by reason of its substantial hardness as mentioned above; (b) the ability of the blow-molded TPE material to transmit axial loads onto the clamp; (c) the relatively inability of such TPE material to accept any high amount of tensile loads without permanent deformation; and (d) the tendency of the TPE blow-molded polymer material to cold flow under the clamp. For example, the compression set of du Pont "Hytrel" tested in accordance with ASTM-D395 method A, at 100.degree. C. is 8%. However, rubber elastomers are normally tested under ASTM-D395 method B, which measures compression set under constant deflection. Such 8% compression set occurs at only about 9% strain and this would be translated as almost a 100% compression set by ASTM-D395 method B. This explains why, in service, most of the problems of sealing blow-molded TPE convoluted boots is that of the replacement of a clamp which has come loose.